The present invention relates to a shock absorbing and pressure reducing insole as that comprises a top foil and a bottom foil joined with first joints along a closed path to provide at least one enclosed cavity, which is filled with at least one fluid, wherein in said enclosed cavity, additional joints are provided. The invention also relates to a process of manufacturing as well as to use of an insole.
A larger number of insoles for footwear are known, where the insole is filled with a fluid, for example gas, liquid or gel. Typically, the insole is manufactured by joining, for example welding or gluing, two foils together along the edge of the insole. Thus, an enclosed cavity is produced which is filled with fluid before or during the joining.
Apart from the joint along the edge, the insole can be provided with additional joints in a particular pattern in order to obtain a massaging or pressure reducing effect.
Such soles are described in international patent application WO 94/23603 and in U.S. Pat. Nos. 4,123,855, 5,778,561, 5,979,086, 4,567,677 and 5,067,255. These massaging insoles are characterised in that one or several liquid cavities are provided extending from the rear of the insole to the front of the insole. The massaging effect arises as a result of the movement of the liquid in-between the heel area and the area under the forefoot as the load on the foot is changed. These soles may be provided with joint patterns designed to obstruct the movement of the liquid, which prolongs the response time of the sole, thus, creating a shock absorbing effect. Furthermore, joints on the insole under the middle of the foot prevent the liquid from gathering at this particular place. The disadvantage of these soles is that a continued load on the heel or forefoot will cause the liquid to flow to the opposite end of the insole, thus, removing the supporting liquid from under the heel and forefoot, respectively.
In order to maintain the liquid support under the heel and forefoot, respectively, an insole has been developed and described in U.S. Pat. No. 4,115,934, in which an insole has been provided with smaller cavities under the heel and under the forefoot. However, such a construction has great disadvantages. Through a load placed such on a cavity, which for example is established centrally under the heel, the liquid will be displaced from the centre to the periphery of the cavity. This principle is not appropriate for thin insoles because all the liquid is displaced from the middle of the cavity to the periphery due to loading. This effect is increasingly significant by long term use, as a repeated load causes so-called creep of the foil material, which results in an easier displacement of the liquid to the periphery of the cavity. Consequently, load by the heel will cause the absence of liquid under the heel. This effect can be counteracted by using very thick insoles, where the cavities contain a large amount of fluid, or where part of the liquid is substituted by a sponge material as in U.S. Pat. No. 5,313,717. However, thick insoles can be difficult to fit into existing footwear. Furthermore, a high, liquid filled insole diminishes the support of the foot by the footwear.
Another disadvantage is that load by the heel causes the liquid to flow from the middle of the heel area to the periphery of the heel cavity within a very short time, whereby the shock absorption is limited considerably. Also, the well known long-term problem of creep of the material has the effect that both shock absorption and pressure reduction decrease substantially with time. In addition to this, the displacement of the liquid to the periphery of the cavity causes problems for larger supporting areas under for example the heel, because that peripheral area also extends across the foot close to the heel bone, where, consequently, a bead of liquid will press up against the tendons and muscles of the foot, which is very uncomfortable and painful. The same effect will arise under the forefoot, where the liquid bead will settle itself especially in the transitional area between the sole of the foot and the toes. Therefore, commercially available insoles only have cavities with very limited supporting areas.
There is a substantial demand for large pressure reducing surfaces in footwear, especially within the orthopaedic field, for example where an effective relief of the entire heel area is necessary in the case of heel spur. Correspondingly, flatfootedness of the forefoot is best solved by a large pressure reducing surface. Furthermore, with shock absorption being a function of collision time and collision area, a large surface will provide a better shock absorption.